aparihāna-dhamma: 'incapable of relapse', or 'of falling away', namely, with regard to deliverance from some or all fetters of existence (s. saṃyojana). Thus all Noble Disciples are called, i.e. all those who have attained  any of the 4 Noble Paths to holiness (s. ariyapuggala). With regard to the absorptions (jhāna, q.v.), anyone is called 'unrelapsable' who has attained  full mastery over the absorptions. See A. VI, 62; Pug. 6. Cf. akuppa-dhamma.

Buddhist Dictionary, Manual of Buddhist Terms and Doctrines, by NYANATILOKA MAHATHERA